Clutch assemblies are commonly used in transmissions and transfer cases for transferring rotational power between two shafts. Typically, this rotational power is transferred through the use of a clutch pack. The clutch pack has a first set of clutch plates which is splined to one shaft and a second set of clutch plates which is splined to another shaft. When the clutch pack is fully compressed, the clutch plates will rotate together thereby causing the two shafts to rotate together.
The clutch pack can also be compressed so as to allow for slip to occur between the first set of clutch plates and the second set of clutch plates, varying the amount of rotational power transferred through the clutch pack, and the two shafts, thereby providing improved vehicle dynamic control. Also, the slip between the first set of clutch plates and the second set of clutch plates can be set to a specific torque level to transfer a specific amount of torque between the two shafts. These clutch packs can be actuated in a number of ways.
One way is to use what is commonly known as a “ball ramp assembly,” which usually consists of a base plate having a series of recesses which support a set of load transferring balls, and a cam plate which also has a series of recesses for supporting the load transferring balls. When the base plate and cam plate are rotated relative to one another they will separate and the distance between them will increase, which will apply force to the clutch pack. One way to actuate the ball ramp assembly is through the use of an electromagnetic clutch. If an electromagnetic clutch is used, the base plate and the cam plate will rotate about the axis in which the shafts rotate. Once the electromagnetic clutch causes relative rotation between the cam plate and the base plate, the load transferring balls will roll in the recesses in the cam plate and the base plate, causing the cam plate to translate axially along the axis about which the shafts rotate, thereby applying force to the clutch pack.
The electromagnetic clutch is usually mounted to the housing and is actuated when electric current is applied to an electromagnetic coil. While using an electromagnetic coil to actuate the ball ramp assembly has proven to be effective, using the electromagnetic coil does not always coincide with various packaging requirements. Also, some applications require a “preemptive” feature for increasing the speed of the engagement of the clutch pack. A preemptive feature includes the use of a solenoid, or other electronic device, for preemptively engaging the clutch pack prior to engagement by the ball ramp assembly. Incorporating a preemptive feature improves overall response time.
However, clutch packs having an electromagnetic clutch do not allow for the incorporation of a preemptive feature because the operation of an electromagnetic clutch requires relative rotation between the base plate and the cam plate for engagement, whereas a clutch pack having a preemptive feature requires that either the base plate or cam plate remain stationary.
Accordingly, there exists a need for an improved way to actuate a ball ramp assembly to engage a clutch pack.